1. Field of the Invention
The present invention relates to a method of phase shift measurement, method of phase shift correction, and magnetic resonance imaging (MRI) apparatus. More particularly, the invention relates to a method of measuring the phase shift of echos caused by the influence of the eddy current and residual magnetization attributable to the preceding encode gradient, etc., a method of correcting the phase shift of echos, and an MRI apparatus which carries out these methods.
2. Description of the Prior Art
The split echo train method is intended to divide multiple echos of an echo train into former-part echos and latter-part echos and produce a first image (proton weighted image) from the former-part echos and a second image (T2 weighted image) from the latter-part echos.
The pulse sequence of the split echo train method has a large amplitude of the encode gradient on its waveform, with its duration being reduced accordingly, in order to reduce the cycle time.
The same technique is used in the conventional high-speed spin echo (SE) method, i.e., it has a pulse sequence including an encode gradient of a large amplitude and short duration in order to reduce the cycle time.
However, the eddy current emerging in the conductor around the gradient coil increases as the amplitude of gradient pulse increases and its duration decreases. The eddy current effects the increase of the phase shift among echos, and the phase shift produces ghost images in the phase axis direction on the image, i.e., creation of artifacts.
A technique for reducing such ghost images is proposed in publication: Proc. SMR, p. 634, 1995, by R. Scott Hinks et al., in which pre-scan data is sampled without applying the encode gradient at pre-scanning, the sampled data is rendered the first-order Fourier transform along the frequency axis of the K-space thereby to obtain phase data, and the offset phase for the read gradient and inversion pulse P is corrected based on the phase data for the main scanning of imaging.
A method of phase correction process is proposed in publication: Mag. Reso. in Med., pp.632-638, 1995, by Xin Wan, Dennis L. Parker, et al., in which after the main scanning of imaging, correction data is sampled without applying the encode gradient, and the phase correction process is implemented based on the correction data at the calculation of image rearrangement.
MRI apparatus based on the permanent magnet also involve the above-mentioned problem of phase shift caused by the eddy current and, in addition, the problem of phase shift caused by the residual magnetization. Specifically, the residual magnetization emerging in the magnetic rectifying plates, for example, increases and affects the phase shift significantly as the pulse amplitude increases.
However, in the foregoing prior art methods which sample correction data without applying the encode gradient, the correction data does not include the influence of the residual magnetization, and therefore these methods cannot correct the phase shift caused by the influence of the residual magnetization.